Create new set of LB hosts on worker boot

What does this MR do?

Create new set of LB hosts on worker boot

As Rails 6.1 has automatic discard for all AR connection pools, we need to create new connection pools.

#332913 (closed)

To test

1. Configure your config/database.yml for DB Load Balancing:

  load_balancing:
    hosts:
      - "<same host as main host>"
      - "<same host as main host>"

1. Restart GDK.

Observe that the error from #332913 (closed) is resolved

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)